About Additive Manufacturing Process Simulation

Additive manufacturing, also referred to as 3D printing, is a broadly used term to describe industrial processes by which three-dimensional objects are manufactured through:

• a controlled deposition of raw material (typically in powdered, melted, or liquid state); and
• induced transformation into a solid state.

Common additive manufacturing processes (ISO / ASTM52900-15) are described in the table below.

Technique	Powder bed	Binder jetting	Directed energy deposition	Material extrusion	Sheet lamination	Photo polymerization	Material jetting
Description	Thermal energy selectively fuses regions of a powder bed.	A liquid bonding agent is deposited to join powder materials.	A nozzle mounted on a multi-axis arm deposits melted material.	Material is drawn via a nozzle, where it is heated. It is deposited layer by layer.	Sheets of material are bonded to form an object.	Liquid photopolymer is selectively cured by light-activated polymerization.	Droplets of build materials are selectively deposited.
Material Form	Powder	Powder	Powder or wire	"Solid" material	"Solid" material	Liquid resin	Ink
Material	Metal Plastic	Metal Plastic Ceramics	Metal	Plastic Composite	Paper Metal	Plastic (photopolymer resin)	Plastic
Processes Terms	Selective laser sintering (SLS) Selective laser melting (SLM) Electron beam melting (EBM) Direct metal laser sintering (DMLS)	Binder jetting (BJ) Inkjet powder printing Multi jet fusion (MJF)	Laser cladding Direct energy deposition (DED) Laser metal deposition (LMD) Laser engineered net shape (LENS) Laser or electron beam wire deposition	Fusion deposition modeling (FDM)	Laminated object manufacturing (LOM) Paper lamination technology (PLT) Ultrasonic additive manufacturing (UAM)	Stereo lithography (SLA) Digital light processing (DLP)	Photopolymer jetting (PolyJet) Multi jet modeling (MJM)

Additive manufacturing makes it possible to produce complex shapes not subject to the design constraints of more traditional manufacturing methods. Therefore, the functional requirements of a part become the primary focus of the design effort. However, additive manufacturing processes have their own challenges. For example, thermal strains induced by the manufacturing process can produce residual stresses large enough to cause failure during printing or during the in-service life of a part.

Some of the main objectives of an additive manufacturing simulation are to:

• Predict the residual stresses in a part.
• Minimize the gap between the designed and manufactured part through process optimization.
• Evaluate how a manufactured part performs under realistic loading conditions in an assembly with other components.

At the core of the Abaqus/Standard additive manufacturing technology is the toolpath-mesh intersection module—a powerful geometry-based engine that takes process toolpath data as input and intersects it with an arbitrary mesh.

Abaqus/Standard provides two methods for the simulation of an additive manufacturing process: a thermomechanical simulation and an eigenstrain-based simulation.

Abaqus/Standard offers general-purpose simulation capabilities that allow you to define appropriate boundary conditions, loads, interactions, constraints, and material models required to capture the physics of additive manufacturing processes. In addition, special analysis techniques are available for the simulation of additive manufacturing processes that take into account machine information and process parameters, such as laser power, layer thickness, and toolpath. Abaqus also allows you to perform postprocessing simulations and in-service performance validations for printed parts.

The toolpath-mesh intersection module is used to find geometric intersections between various toolpaths used in additive manufacturing processes and a finite element mesh of the part to be manufactured. For example, in a typical metal powder bed simulation, you can define the time-location history of both powdered metal deposition and a heating source (such as a laser) using an event series. The toolpath-mesh intersection module uses the event series data and automatically computes the relevant information required to activate elements and to apply the proper thermal energy to the model.

A thermomechanical simulation consists of a transient heat transfer analysis of thermal loads introduced on a part during the printing process followed by a static structural analysis that is driven by the temperature field from the thermal analysis. The simulation allows exact specification in time and space of processing conditions and gives you precise control over the fidelity of the solution. The simulation is accurate and comprehensive, but it can be computationally expensive with increasing time and spatial (mesh) resolution. For more information, see Thermomechanical Simulation of Additive Manufacturing Processes.

Eigenstrain, or inherent strain, is an engineering concept used to account for all possible sources of permanent (or inelastic) deformation induced by the process. Eigenstrain has long been used for the evaluation of residual stresses from welding operations. An eigenstrain-based simulation of an additive manufacturing process consists of a single static stress analysis of a printing part where a predefined eigenstrain field is applied according to an element activation sequence representative of the process (usually layer by layer). This process results in a distribution of residual stresses and a deformation field that can lead to the overall distortion of the part. This method eliminates the need to obtain detailed machine information; however, it requires additional effort to calibrate eigenstrain values from experiments or detailed process-level thermomechanical simulations. The results from an eigenstrain-based simulation are typically more approximate than a thermomechanical simulation. Generally, the eigenstrain analysis is sufficient to capture distortion and residual stresses but may not capture higher-order deformation modes, such as the buckling that can happen in thin-walled components during printing. For more information, see Eigenstrain-Based Simulation of Additive Manufacturing Processes.

The functionality in Abaqus/Standard for additive manufacturing processes simulation is developed on a user subroutine infrastructure and keyword interface that provides a high degree of control and customization. In addition, a number of special-purpose techniques are available for simulation of common AM processes that do not require you to write user subroutines. These techniques are implemented as "internal" user subroutines in Abaqus using the same user subroutine infrastructure and keyword interface. These special-purpose techniques are accessed by using table collections with string names starting with "ABQ_". Table collections with string names starting with "ABQ_" are reserved for special-purpose techniques and should not be used when programming your own user subroutines.

It is often necessary to simulate additional postprocessing operations performed on a printed part, such as wire electrical discharge machining (EDM) to remove a part from a build plate or to remove support structures, heat treatment, or other subsequent machining processes.

Cutting processes by wire EDM can be modeled using progressive removal of specified elements in the cutting region. Progressive element removal can be achieved by assigning a zero-valued material volume fraction to elements in a progressive element activation definition (see Progressive Element Activation). Alternatively, you can specify elements to remove from the model using the model change technology (see Removing Elements).

You can also apply heat treatment thermal cycles to simulate the reduction of residual stresses in the part due to annealing or other thermally induced phase transformations.

Furthermore, you can apply in-service loading to a printed part to account for residual stress effects from the manufacturing history. Abaqus/Standard offers various options to model part failure and part durability that can be incorporated into subsequent in-service performance validations.

• ISO / ASTM52900-15, "Standard Terminology for Additive Manufacturing-General Principles-Terminology," ASTM International, West Conshohocken, PA, 2015.